Duro's finish in the 2nd half of at San Mames leaves the tie firmly in the stability, with Valencia playing the second leg at home at Mestalla on March 2.
Athletic's Raul Garcia had given them the lead rapidly earlier than half of-time but the 23-time winners of the Spanish Cup have been not able to hold on for the victory.
The winner of the tie will play both Rayo Vallecano or Real Betis inside the final in April. Betis defeated Rayo 2-1 at Vallecas within the first leg on Wednesday night.
Athletic knocked out Real Madrid inside the sector-finals and to win the cup this season could be particularly unique, after they lost two finals in weeks remaining 12 months.
The first defeat was by way of Basque competitors Real Sociedad in a postponed very last from 2020 earlier than they misplaced again to Barcelona inside the 2021 competition a fortnight later.
Athletic are the most successful Copa del Rey membership left inside the tournament given handiest Barcelona can higher their 23 cup victories.
But of the 4 semi-finalists, Valencia have received it most recently once they beat Barca in 2019, under Marcelino, who's now in charge of Athletic.
The home aspect took the lead in the thirty seventh minute when Garcia held off former Barcelona midfielder Ilaix Moriba at the returned put up to move in Iker Muniain's bending go.
Valencia's replacement Maxi Gomez became despatched off at 1/2-time as a feisty 45 minutes ended with a confrontation among gamers from both groups. Gomez had yet to play in the game but will now be unavailable for the second one leg.
There changed into little among the perimeters after the restart and Valencia grabbed an equaliser simply after the hour-mark as Athletic goalkeeper Julen Agirrezabala did well to parry Bryan Gil's end however the ball fell directly to Duro, who bundled in.
